The structural changes associated with the transition of muscle fibers from one physiological state to another have been examined by X-ray diffraction techniques, using radiation from conventional rotating anode generators and synchrotronss in conjuction with electronic, position sensitive, X-ray detectors. The rate constants associated with the turnover of actomyosin cross-bridges have been studied by analyzing the contraction kinetics of skinned muscle fibers under controlled chemical conditions. The mechanical properties of muscle fibers the rigor (ATP-free) state have been analyzed as a function of myofilament lattice spacing to obtain information about the geometry of the actomyosin cross-bridges.